1. Field of the Invention
The invention relates to a thermal control variable-speed circuit, and more particularly, to a thermal control variable-speed circuit without switching noise.
2. Description of the Related Art
Recently, methods for controlling the speed of heat dissipating fan motors were mainly accomplished by pulse width modulation (PWM) signals. Such control methods are illustrated in FIGS. 1 and 2. The principle of PWM controlling methods is to utilize a reference voltage signal Vref and a triangular wave (TW) signal to adjust the duty time. Referring to FIG. 2, Vref1 and Vref2 represent the reference voltage signal Vref under different circumstances. When the reference voltage signal Vref is Vref1, the output PWM signal P is P1 with a corresponding duty time T1. On the other hand, when the reference voltage signal Vref is Vref2, the output PWM signal P is P2 with a corresponding duty time T2. A driving circuit 12 then controls the operating time of a fan motor 13 according to the duty time of the PWM signal P.
For example, at higher ambient temperatures, Vref2 corresponding to the reference voltage signal Vref is produced, and a TW signal is inputted to a PWM signal generating module 11. The PWM signal generating module 11 produces and outputs P2 as the PWM signal P, and the driving circuit 12 controls the duty time of the fan motor 13 according to the duty time T2 of the PWM signal P2. Because the duty time T2 per unit time is longer and thus average duty time of the fan motor 13 is also longer, the rotation speed of the fan motor 13 is faster. Conversely, at lower ambient temperatures, Vref1 corresponding to the reference voltage signal Vref is produced. The PWM signal generating module 11 produces and outputs P1 as the PWM signal P. Then, the driving circuit 12 controls the duty time of the fan motor 13 according to the duty time T1 of the PWM signal P1. Because the duty time T1 per unit time is shorter and average duty time of the fan motor 13 is also shorter, the rotation speed of the fan motor 13 is slower, thereby accomplishing the purpose of controlling the rotation speed of the fan motor 13.
However, such methods for controlling the rotation speed of a fan motor by using PWM signals are prone to the following shortcomings. While the driving circuit 12 is dedicated to constantly receive PWM signals P for controlling the fan motor, the operation of switching between high and low levels of the PWM signal P causes switching noises that further produce other mild noises. When the fan motor operates at high rotation speeds, the noise level is below that created by the fan motor. However, when the rotation speed of the fan motor decreases, the noises produced become apparent and cannot meet the requirements of system manufacturers. In addition, in order to ensure that the driving circuit 12 and the fan motor 13 are appropriately controlled by the PWM signals P, it is necessary to constrain the duty time of the PWM signals P within a range of 30% to 85%. However, the rotation speed of the fan motor will then be in a limited range and thus the ambient temperature cannot be adjusted as desired.
Therefore, it is an important task of the invention to avoid the switching noises during the rotation of the fan motor, and to broaden the controllable range of the rotation speed of the fan motor.